Polyimide

ABSTRACT

A polyamic acid is herein disclosed which contains a structural unit represented by the formula  2a! ##STR1## wherein m is an integer of 1 to 20; n is an integer of 1 to 2; and R 2  is an alkyl group or an alkoxy group having 1 to 20 carbon atoms. Furthermore, a polyamic acid composition is also disclosed which comprises the polyamic acid containing the structural unit represented by the formula  2a! and another polyamic acid containing a structural unit other than mentioned above. By heating the polyamic acid or the polyamic acid composition at a low temperature for a short time, a liquid crystal alignment film can be obtained therefore.

BACKGROUND OF THE INVENTION

1. (i) Field of the Invention

The present invention relates to a polyamic acid, a polyimide, apolyamic acid composition, a polyimide composition and a liquid crystalalignment film having a side chain containing a liquid crystal formationfactor such as a phenylcyclohexyl group or a phenylbicyclohexyl group,i.e., a mesogen group.

2. (ii) Description of the Related Art

Polyimides are good plastics having the highest heat resistance ofcommercially available plastics, and they are excellent in impactresistance, dimensional stability, electrical properties and wearresistance and scarcely change their characteristics by temperature. Bythe utilization of such features, the polyimides have been used asinsulating materials, sealing material, print substrates in electricaland electronic fields, materials for aerospace and material formachines. In recent years, much attention is paid to the polyimides asmaterials for alignment films of liquid crystal displays, and many kindsof polyimide alignment films have been developed.

In each of the liquid crystal displays which have been used in watchesand electronic calculators, a twisted nematic (hereinafter abbreviatedto "TN") mode in which an orientating direction of a nematic liquidcrystal molecule is twisted 90° is mainly employed between a pair ofupper and lower electrode substrates. Furthermore, a super-twistednematic (hereinafter abbreviated to "STN") mode in which the twist angleof the liquid crystal is in the range of 180° to 270° has also beenprevalent of late, and the liquid crystal displays having the STN modehave been mass-produced as large displays for word processors, personalcomputers and the like. In the alignment films which can be used inthese liquid crystal displays, the mere alignment of the liquid crystalmolecules is insufficient, and in order to improve responsibility and tosecure bistability, a pretilt angle of 1° to 4° in terms of the TN modeor 4° to 8° in terms of the STN mode must be taken between a substratesurface and the liquid crystal molecule. Nowadays, for the displayshaving excellent contrast and visual dependence, a super-twistedbirefringence effect (hereinafter abbreviated to "SBE") mode has beendeveloped, and in this mode, a high pretilt angle of 20° to 30° isrequired.

The demand of the color liquid crystal displays increases, but one ofthe color display systems which have now widely been used is a colorfilter system, and the utilization of this system has spread to liquidcrystal color televisions and displays for personal computers. Thissystem is a system in which a color filter is interposed between asubstrate and transparent electrodes. In this system, the substrateprovided with the transparent electrodes and the color filter is coatedwith a polyamic acid, followed by drying and baking, to form a polyimidefilm, but the dye type color filter is relatively poor in heatresistance, and hence the filter possibly brings about color failure anddeterioration by the baking. Therefore, the attachment of the colorfilter is preferably accomplished by the baking at a lowest possibletemperature.

Japanese Patent Application Laid-open No. Hei 4-100020 has disclosed analignment film of a polyimide obtained from a diamino compound having astraight-chain alkyl ester on a side chain and3,3',4,4'-biphenyltetracarboxylic dianhydride, and a liquid crystaldisplay using the same. With regard to the pretilt angle of thisalignment film, however, any detailed description is not made.

On the other hand, in Japanese Patent Application Laid-open Nos. Hei3-179322, Hei 3-179323 and Hei 5-27244, there have been disclosedpolyimide liquid crystal alignment films obtained from a diamine havinga mesogen group on a side chain as a monomer. In these polyimides,however, the baking at 230° to 450° C. for 1 to 2 hours is required fortheir formation. Thus, when the dye color filters having the poor heatresistance are attached, there is a fear that the color failure anddeterioration of the filters occur by the baking.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above-mentionedproblems, i.e., to provide a polyamic acid and a polyamic acidcomposition which can produce a polyimide by baking at a low temperaturefor a short period of time and which do not give rise to the colorfailure of a color filter or the like at the time of the formation of aliquid crystal cell.

Another object of the present invention is to provide a polyimide and apolyimide composition which have excellent liquid crystal alignmentproperties and which can freely form a liquid crystal alignment filmhaving a necessary pretilt angle in a wide range.

Still another object of the present invention is to provide a liquidcrystal display equipped with the liquid crystal alignment film.

The present inventors have intensively investigated a polyimide whichcan be obtained by reacting a diamine component having a mesogen groupsuch as a phenylcyclohexyl group or a phenylbicyclohexyl group via aspacer comprising a flexible alkyl chain on a side chain with atetracarboxylic dianhydride, and patent applications have already beenmade (Japanese Patent Application Nos. Hei 6-173269 and Hei 6-234162).As a result of further investigations, it has been found that apolyimide alignment film having excellent liquid crystal alignmentproperties can be formed from a polyamic acid obtained by reacting atetracarboxylic dianhydride with two kinds of diamine componentscontaining a diamine component which possesses a side chain having amesogen group via a spacer comprising a flexible alkyl chain, or apolyamic acid composition comprising a polyamic acid obtained byreacting a tetracarboxylic dianhydride with a diamine component having amesogen group on a side chain via a spacer comprising a flexible alkylchain and a polyamic acid obtained by reacting another diamine componentwith a tetracarboxylic dianhydride.

That is to say, the constitutions of the present invention comprise thefollowing paragraphs 1 to 15, respectively.

1. A polyamic acid having a logarithmic viscosity number of 0.2 to 1.0g/dl (an N-methyl-2-pyrrolidone solvent, concentration=0.5 g/dl, 30° C.)and containing a structural unit represented by the formula 2a! ##STR2##wherein m is an integer of 1 to 20; n is an integer of 1 to 2; and R² isan alkyl group or an alkoxy group having 1 to 20 carbon atoms.

2. A polyamic acid having a logarithmic viscosity number of 0.2 to 1.0g/dl (an N-methyl-2-pyrrolidone solvent, concentration=0.5 g/dl, 30° C.)and comprising 5 mol % or more of a structural unit represented by theformula 2a! and less than 95 mol % of a structural unit represented bythe formula 1a! ##STR3## wherein m is an integer of 1 to 20; n is aninteger of 1 to 2; R¹ is an alkyl group having 1 to 20 carbon atoms; andR² is an alkyl group or an alkoxy group having 1 to 20 carbon atoms.

3. A polyimide in which a logarithmic viscosity number of a polyamicacid as a precursor is in the range of 0.2 to 1.0 g/dl (anN-methyl-2-pyrrolidone solvent, concentration=0.5 g/dl, 30° C.) andwhich contains a structural unit represented by the formula 2! ##STR4##wherein m is an integer of 1 to 20; n is an integer of 1 to 2; and R² isan alkyl group or an alkoxy group having 1 to 20 carbon atoms.

4. A polyimide in which a logarithmic viscosity number of a polyamicacid as a precursor is in the range of 0.2 to 1.0 g/dl (anN-methyl-2-pyrrolidone solvent, concentration=0.5 g/dl, 30° C.) andwhich comprises 5 mol % or more of a structural unit represented by theformula 2! and less than 95 mol % of a structural unit represented bythe formula 1! ##STR5## wherein m is an integer of 1 to 20; n is aninteger of 1 to 2; R¹ is an alkyl group having 1 to 20 carbon atoms; andR² is an alkyl group or an alkoxy group having 1 to 20 carbon atoms.

5. The polyimide according to the above-mentioned paragraph 4 which isobtained by reacting an alkyl 3,5-diaminobenzoate represented by theformula 3!, 4-(trans-4-alkyl-(di)cyclohexyl)phenoxyalkyl!3,5-diaminobenzoate represented by the formula 4! and3,3',4,4'-biphenyltetracarboxylic dianhydride represented by the formula5! in a solvent, and then heating a produced polyamic acid solution:##STR6## wherein m is an integer of 1 to 20; n is an integer of 1 to 2;R¹ is an alkyl group having 1 to 20 carbon atoms; and R² is an alkylgroup or an alkoxy group having 1 to 20 carbon atoms.

6. A polyamic acid composition comprising a polyamic acid containing astructural unit of the formula A! and a polyamic acid containing astructural unit of the formula B! ##STR7## wherein m is an integer of 1to 20; n is an integer of 1 to 2; R² is an alkyl group or an alkoxygroup having 1 to 20 carbon atoms; A is a divalent organic group; B is atetravalent organic group; and each of p and q is a value which meetsthat a logarithmic viscosity number of the polyamic acid is in the rangeof 0.2 to 1.0 g/dl (an N-methyl-2-pyrrolidone solvent, concentration=0.5g/dl, 30° C.).

7. The polyamic acid composition according to the above-mentionedparagraph 6 in which the formula B! is a structural unit represented bythe formula B'! ##STR8## wherein R¹ is an alkyl group having 1 to 20carbon atoms; B is a tetravalent organic group; and q is a value whichmeets that a logarithmic viscosity number of the polyamic acid is in therange of 0.2 to 1.0 g/dl (an N-methyl-2-pyrrolidone solvent,concentration=0.5 g/dl, 30° C.).

8. A polyimide composition comprising a structural unit represented bythe formula S1! and a structural unit represented by the formula S2!##STR9## wherein m is an integer of 1 to 20; n is an integer of 1 to 2;R² is an alkyl group or an alkoxy group having 1 to 20 carbon atoms; Ais a divalent organic group; B is a tetravalent organic group; and eachof α and β is a value which meets that a logarithmic viscosity number ofa polyamic acid as a precursor is in the range of 0.2 to 1.0 g/dl (anN-methyl-2-pyrrolidone solvent, concentration=0.5 g/dl, 30° C.).

9. The polyimide composition according to Claim 8 which is obtained bymixing a polyamic acid obtained by reacting a diamino compoundrepresented by the formula S3! (not containing a structural unit of theformula 4!) with a tetracarboxylic dianhydride represented by theformula S4! in a solvent with a polyamic acid obtained by reacting adiamino compound represented by the formula 4! with a tetracarboxylicdianhydride represented by the formula S4! in a solvent, and thenheating the mixture to carry out dehydration and ring closure: ##STR10##wherein A is a divalent organic group; B is a tetravalent organic group;m is an integer of 1 to 20; n is an integer of 1 to 2; and R² is analkyl group or an alkoxy group having 1 to 20 carbon atoms.

10. The polyimide composition according to Claim 8 wherein the formulaS1! is a structural unit represented by the formula 1'! and the formulaS2! is a structural unit represented by the formula 2'!: ##STR11##wherein m is an integer of 1 to 20; n is an integer of 1 to 2; R¹ is analkyl group having 1 to 20 carbon atoms; R² is an alkyl group or analkoxy group having 1 to 20 carbon atoms; and each of x and y is a valuewhich meets that a logarithmic viscosity number of a polyamic acid as aprecursor is in the range of 0.2 to 1.0 g/dl (an N-methyl-2-pyrrolidonesolvent, concentration=0.5 g/dl, 30° C.).

11. A liquid crystal alignment film comprising the polyimide describedin the above-mentioned paragraph 3.

12. A liquid crystal alignment film comprising the polyimide compositiondescribed in the above-mentioned paragraph 8.

13. A liquid crystal display equipped with the liquid crystal alignmentfilm described in the above-mentioned paragraph 11.

14. A liquid crystal display equipped with the liquid crystal alignmentfilm described in the above-mentioned paragraph 12.

15. 4-(trans-4-alkyl-(di)cyclohexyl)phenoxyalkyl!3,5-diaminobenzoaterepresented by the formula 4! ##STR12## wherein m is an integer of 1 to20; n is an integer of 1 to 2; and R² is an alkyl group or an alkoxygroup having 1 to 20 carbon atoms.

Incidentally, the tetravalent organic group is a tetravalent grouphaving an aromatic ring, and the divalent organic group A is a divalentgroup having an aromatic ring. Preferably, each of these groups has acarboxyl group or an alkylcarbonyl group on a side chain. Thecyclohexylene group is a trans form.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an IR spectrum of a polyimide obtained in Example 1.

FIG. 2 shows an IR spectrum of a polyimide composition obtained inExample 8.

FIG. 3 shows an IR spectrum of a polyimide obtained in Example 12.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A polyamic acid of the present invention contains a repeating structuralunit represented by the formula 2a!. That is to say, the polyamic acidsof the present invention are a condensation polymer comprising arepeating structural unit represented by the formula 2a!, and aco-condensation polymer comprising a repeating structural unitrepresented by the formula 2a! and a repeating structural unitrepresented by the formula 1a!.

In the structural unit represented by the formula 2a!, m is an integerof 1 to 20, preferably 3 or more, more preferably 6 or more. If m is 3or less, the flexibility of the polymer in a side chain directiondeteriorates, and thus at the time of imidation, high-temperature bakingis necessary.

In the structural unit represented by the formula 1a!, R¹ is an alkylgroup having 1 to 20 carbon atoms, preferably 3 or more carbon atoms,more preferably 6 or more carbon atoms. If the number of the carbonatoms is 3 or less, the flexibility of the polymer in a side chaindirection deteriorates, so that low-temperature setting properties tendto deteriorate.

The polyamic acid of the present invention can be obtained in the stateof a polyamic acid solution by carrying out the condensationpolymerization reaction of4-(trans-4-alkyl-(di)cyclohexyl)phenoxyalkyl!3,5-diaminobenzoaterepresented by the formula 4! and 3,3',4,4'-biphenyltetracarboxylicdianhydride (BPDA) represented by the formula 5! in a solvent, or thecondensation reaction of two kinds of diamines of an alkyl3,5-diaminobenzoate represented by the formula 3!,4-(trans-4-alkyl-(di)cyclohexyl)phenoxyalkyl!3,5-diaminobenzoaterepresented by the formula 4! and 3,3',4,4'-biphenyltetracarboxylicdianhydride (BPDA) represented by the formula 5! in a solvent. Thereaction is carried out under anhydrous conditions at a temperature of-10° to 30° C. in a solvent such as dimethylacetamide (DMAc),N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylsulfoxide (DMSF), sulfolane, butyrolactone, cresol, phenol, ahalogenated phenol, cyclohexanone, dioxane or tetrahydrofuran,preferably in the NMP solvent.

A polyimide of the present invention can be obtained by coating asubstrate with the above-mentioned polyamic acid solution, and thenthermally treating the same at 100° to 450° C., but the presentinvention is characterized by imidating the polyamic acid solution at alow temperature of 100° to 200° C. for a short period of time.

A polyamic acid composition of the present invention comprises apolyamic acid containing a structural unit of the formula (A) andanother polyamic acid containing a structural unit of the formula (B).In the structural unit represented by the formula (A), m is an integerof 1 to 20, preferably 3 or more, more preferably 6 or more. If m is 3or less, the flexibility of the polymer in a side chain directiondeteriorates, and thus at the time of the imidation, high-temperaturebaking is necessary.

The polyamic acid composition of the present invention is obtained bymixing the solution of a polyamic acid (A) obtained by reacting adiamino compound represented by the formula 4! with a tetracarboxylicdianhydride represented by the formula S4! in a solvent with thesolution of a polyamic acid (B) obtained by reacting a diamino compoundrepresented by the formula S3! (not containing the structural unit ofthe formula 4!) with a tetracarboxylic dianhydride represented by theformula S4! in a solvent. A composition ratio of the above-mentioned twokinds of polyamic acids is preferably such that the polyamic acid (A) is5 mol % or more and the polyamic acid (B) is less than 95 mol %.

The above-mentioned condensation reaction of the diamino compound withthe tetracarboxylic dianhydride is carried out under anhydrousconditions at a temperature of -10°to 30° C. in a solvent such as DMAc,N-methyl-NMP, DMF, dimethyl sulfoxide (DMSO), sulfolane, butyrolactone,cresol, phenol, a halogenated phenol, cyclohexanone, dioxane andtetrahydrofuran, preferably in the NMP solvent.

Examples of the diamino compound represented by the formula S3! (notcontaining the structural unit of the formula 4!) include the followingcompounds, but they are not always restrictive.

Typical examples of aromatic diamines include 4,4'-diaminodiphenylether, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylsulfone,3,3'-diaminodiphenylsulfone, 4,4'-diaminodiphenyl sulfide,4,4'-di(m-aminophenoxy)diphenylsulfone,4,4'-di(p-amonophenoxy)diphenylsulfone, o-phenylenediamine,m-phenylenediamine, p-phenylenediamine, benzidine,3,3'-diaminobenzophenone, 4,4'-diaminobenzophenone,4,4'-diaminodiphenyl-2,2'-propane, 1,5-diaminonaphthalene,1,8-diaminonaphthalene, 3,4'-diaminodiphenyl ether,4,4'-bis(4-aminophenoxy)biphenyl,2,2-bis(4-(4-aminophenoxy)phenyl}hexafluoropropane,1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene,1,3-bis(3-aminophenoxy)benzene,4,4'-diamino-3,3'-diethyl-5,5'-dimethyldiphenylmethane,4,4'-diamino-3,3',5,5'-tetramethyldiphenylmethane, 1,4-diaminotoluene,m-xylenediamine, 2,2'-dimethylbenzidine and the like; and examples ofalicyclic diamines include 1,4-diaminocyclohexane, isophoronediamine andthe like. In addition, there are also preferably used diamines in whichan aromatic ring or an alicyclic ring of each of the above-mentioneddiamines has a carboxyl group or an alkylcarbonyl group as a side chain.

Examples of the tetracarboxylic dianhydride represented by the formulaS4! include the following compounds, but they are not alwaysrestrictive.

Typical examples of aromatic tetracarboxylic dianhydrides includepyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride,2,2',3,3'-biphenyltetracarboxylic dianhydride,2,3,3',4'-biphenyltetracarboxylic dianhydride,3,3',4,4'-benzophenonetetracarboxylic dianhydride,2,3,3',4'-benzophenonetetracarboxylic dianhydride,2,2',3,3'-benzophenonetetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl) ether dianhydride,bis(3,4-dicarboxyphenyl)sulfone dianhydride,1,2,5,6-naphthalenetetracarboxylic dianhydride,2,3,6,7-naphthalenecarboxylic dianhydride,2,2-bis(3,4-dicarboxyphenyl)tetrafluoropropane dianhydride and the like;and typical examples of alicyclic tetracarboxylic dianhydrides includecyclobutanetetracarboxylic dianhydride, methylcyclobutanetetracarboxylicdianhydride and the like.

Examples of the solvent include N-methyl-2-pyrrolidone,N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide,tetramethyl urea, pyridine, hexamethylphosphoramide, methylformamide,N-acetyl-2-pyrrolidone, 2-methoxyethanol, 2-ethoxyethanol,2-butoxyethanol, diethylene glycol monomethyl ether, diethylene glycolmonoethyl ether, diethylene glycol monobutyl ether, cyclopentanone,cyclohexanone, cresol, γ-butyrolactone, isophorone,N,N-diethylacetamide, N,N-diethylformamide,N,N-dimethylmethoxyacetamide, tetrahydrofuran, N-acetyl-2-pyrrolidone,N-methyl-ε-caprolactam and tetrahydrothiophene dioxide {sulpholane}.Furthermore, the above-mentioned reaction can be carried out in a mixedsolvent obtained by mixing two or more of the above-mentioned organicsolvents. In addition, the above-mentioned preferable organic solvents,prior to use, can be diluted with another aprotic (neutral) organicsolvent such as an aromatic, an alicyclic or an aliphatic hydrocarbon,its chlorinated derivative (e.g., benzene, toluene, a xylene,cyclohexane, pentane, hexane, petroleum ether or methylene chloride), ordioxane.

The alkyl 3,5-diaminobenzoate represented by the formula 3! which is oneof the diamino compounds regarding the preparation of the polyamic acidand the polyamic acid composition of the present invention can besyntherized in accordance with the following procedure ##STR13## whereinR² is an alkyl group having 1 to 20 carbon atoms.

That is to say, an alkyl alcohol and 3,5-dinitrobenzoyl chloride arecondensed in the presence of triethylamine to obtain an alkyl3,5-dinitrobenzoate, and this compound is then subjected to catalyticreduction in the presence of a palladium-carbon catalyst, therebysynthesizing the alkyl 3,5-diaminobenzoate.

The 4-(trans-4-alkyl-(di)cyclohexyl)phenoxyalkyl!3,5-diaminobenzoaterepresented by the formula 4!, i.e., a diamine component having amesogen group on a side chain which takes part in the preparation of thepolyamic acid and the polyamic acid composition of the present inventioncan be synthesized in accordance with the following procedure ##STR14##wherein m is an integer of 1 to 20; n is an integer of 1 to 2; R² is analkyl group or an alkoxy group having 1 to 20 carbon atoms; and X is ahalogen.

That is to say, in the first place, a 4-(trans-4-alkyl-cyclohexyl)phenolor a 4-(trans-4-alkyl-dicyclohexyl)phenol and an αω-alkylenehalohydrinare condensed at 100°to 150° C. in the presence of excess potassiumcarbonate in a solvent such as dimethylacetamide (DMAc),N-methyl-2-pyrrolidone (NMP) or dimethylformamide (DMF) to obtain anphenoxyalkyl alcohol. In succession, this compound and3,5-dinitrobenzoyl chloride are condensed in the presence oftriethylamine, thereby obtaining4-(trans-4-alkylcyclohexyl)phenoxyalkyl!3,5-dinitrobenzoate or4-(trans-4-alkyl-dicyclohexyl)phenoxyalkyl!3,5-dinitrobenzoate. Finally,this compound was subjected to catalytic reduction in the presence of apalladium-carbon catalyst to synthesize4-(trans-4-alkylcyclohexyl)phenoxyalkyl!3,5-diaminobenzoate or4-(trans-4-alkyl-dicyclohexyl)phenoxyalkyl!3,5-diaminobenzoate.

The polyimide composition of the present invention can be obtained bybaking the above-mentioned polyamic acid composition solution at about200° C. A preferable method can be used which comprises coating asubstrate with the polyamic acid composition solution, and then heatingit to carry out dehydration reaction, whereby the polyimide compositionis formed on the substrate. When the polyamide composition of thepresent invention is heated at a low temperature of 100° to 200° C. fora short time, a polyimide composition can be obtained.

A composition ratio between the polyimide containing a structural unitrepresented by the formula 1'! and the polyimide containing a structuralunit represented by the formula 2'! in the polyimide composition of thepresent invention is preferably such that the former is 95 to 0 mol %and the latter is 5 to 100 mol %.

The substrate which can be used in a liquid crystal alignment film and aliquid crystal display is usually a glass substrate, and on thesubstrate, electrodes, concretely, transparent electrodes of ITO (indiumoxidetin oxide) or tin oxide are formed. Furthermore, in order toprevent an alkali from oozing from the substrate, an insulating film oran undercoat film such as a color filter or a color filter overcoat maybe formed between the substrate and the electrodes, and on theelectrodes, an insulating film or an overcoat film such as a colorfilter may be formed. In addition, an active element such as a TFT (thinfilm transister) or an MIM (metal-insulator-metal) may be formed on theelectrodes. As the electrodes, the undercoat and other members in aliquid crystal cell, there can be used members in conventional liquidcrystal cell.

The formation of the polyimide, the polyimide composition of the presentinvention or the liquid crystal alignment film on the substrate can beaccomplished in accordance with the following procedure. First, thepolyamic acid or the polyamic acid composition of the present inventionis dissolved in a solvent such as NMP, DMAc, DMF, DMSO, butyl cellosolveor ethyl carbitol to prepare a 0.1 to 30 wt % solution, and thissolution is then applied onto the substrate by a brush applicationmethod, an immersion method, a spin coating method, a spray method or aprinting method. Afterward, baking is done at 100° to 450° C.,preferably 180° to 220° C. to carry out a dehydration/cyclizationreaction, thereby forming the film of the polyimide or the polyimidecomposition. If the surface of the substrate is treated with a silanecoupling agent prior to the application of the solution to form the filmof the polyimide or the polyimide composition thereon, adhesiveproperties between the film and the substrate can be improved.Afterward, the surface of this film is rubbed in one direction with acloth or the like to obtain the liquid crystal alignment film.

A cell is formed by the use of the substrate for a liquid crystaldisplay, and a liquid crystal is then injected into the cell. Next, aninjection opening is closed to form the liquid crystal display. As thisliquid crystal to be injected, there can be used various liquid crystalssuch as a usual nematic liquid crystal and a liquid crystal to which adichroic dye is added.

The liquid crystal alignment film of the present invention is excellentin liquid crystal alignment properties, and so a pretilt angle can beoptionally selected in a wide range of 5° to 45° by suitably selecting aratio of components of the polyamic acid or the polyamic acidcomposition of the present invention.

The liquid crystal display of the present invention is characterized bybeing equipped with the liquid crystal alignment film which is excellentin liquid crystal alignment properties and in which the pretilt anglecan be optionally controlled in a wide range of 5° to 45°, i.e., theliquid crystal alignment film regarding the present invention. Ingeneral, the liquid crystal display is constituted of the substrate, avoltage application means, the liquid crystal alignment film, a liquidcrystal layer and the like.

The polyimide or the polyimide composition of the present invention canbe prepared by baking the polyamic acid or the polyamic acid compositionof the present invention at a low temperature for a short period oftime, and the decomposition temperature of the polyimide is 350° C. ormore and that of the polyimide composition is 320° C. or more, whichmeans that the polyimide or the polyimide composition of the presentinvention can be used at high temperatures. Furthermore, this polyimideor polyimide composition is excellent in adhesive properties to theglass substrate and liquid crystal alignment properties. In addition,the pretilt angle of the liquid crystal alignment film can be optionallycontrolled in a wide range of about 5° to 45° by suitably changing aratio of copolymerization and a ratio of components in the composition,and therefore the polyimide or the polyimide composition of the presentinvention are useful as materials for the alignment films of liquidcrystal cells of TN, STN and SBE.

EXAMPLES

Next, the present invention will be described in more detail withreference to examples, but the scope of the present invention should notbe limited to these examples at all.

Physical properties in the examples were measured in the followingmanners.

Decomposition temperature (Td): Measurement was made at a temperaturerise velocity of 10° C. per minute by the use of a differential thermalweight simultaneous measuring device (TG/DTA-220 type, made by SeikoElectronic Industry Co., Ltd.), and a temperature at which weightreduction had reached 5% was regarded as Td.

Logarithmic viscosity number: This was measured at 30° C. at aconcentration of 0.5 g/dl in an NMP solvent by the use of a Ubbelohde'sviscometer.

Pretilt angle: This was measured by the use of a magnetic fieldelectrostatic capacity method.

Example 1

(1) Preparation of stearyl 3,5-diaminobenzoate (in the formula 3!, R¹═C₁₈ H₃₇)

(a) Preparation of stearyl 3,5-dinitrobenzoate

In a 500-ml three-necked flask equipped with a stirrer was placed 200 mlof tetrahydrofuran (THF), and 13.5 g (0.050 mol) of stearyl alcohol and8.4 ml of triethylamine were added thereto, followed by stirring at 0°C. Afterward, a solution obtained by dissolving 11.5 g (0.050 mol) of3,5-dinitrobenzoyl chloride in 50 ml of THF was added dropwise to theflask over 30 minutes, and under this state, reaction was carried outfor 6 hours. After the completion of the reaction, this solution waspoured into 1 liter of water, and extraction was then made with 1.5liters of ethyl acetate. In succession, the resultant organic layer waswashed with 3N hydrochloric acid three times, a saturated sodiumbicarbonate solution three times and water. The resultant ethyl acetatelayer was dried over anhydrous sodium sulfate, and the solvent was thendistilled off under reduced pressure. Next, the resultant crystal wasrecrystallized from ethyl acetate twice to obtain 18.1 g (0.039 mol) ofstearyl 3,5-dinitrobenzoate. Its melting point was in the range of 75.9°to 76.7° C. The structure of this compound was confirmed by an infraredabsorption spectrum (IR) and NMR.

(b) Preparation of stearyl 3,5-diaminobenzoate: In 300 ml of ethylacetate was dissolved 8.0 g (0.017 mol) of stearyl 3,5-dinitrobenzoate,and 1.0 g of 5% palladium-carbon was then added thereto and catalyticreduction is carried out at ordinary temperature under atmosphericpressure. After the completion of the reaction, the catalyst was removedby filtration and the solvent was distilled off under reduced pressureto obtain a crystal. The thus obtained crystal was recrystallized fromn-heptane/ethyl acetate twice to obtain 4.48 g (0.011 mol) of stearyl3,5-diaminobenzoate. The melting point of this compound was in the rangeof 81.8° to 82.6° C.

(2) Preparation of4-(trans-4-ethylcyclohexyl)phenoxypropyl!3,5-diaminobenzoate (however,in the formula 4!, m=3, n=1 and R² ═C₂ H₅)

(a) Preparation of 4-(trans-4-ethyl-cyclohexyl)phenoxypropanol

In a 1-liter three-necked flask equipped with a condenser and a stirrerwere placed 500 ml of dimethylformamide, 20.4 g (0.10 mol) of4-(trans-4-ethylcyclohexyl)phenol and 41.4 g (0.30 mol) of potassiumcarbonate, followed by stirring at room temperature. Afterward, 18.1 g(0.13 mol) of 3-bromo-1-propanol was added thereto, and the solution wasthen vigorously stirred at 100° C. After reaction for 10 hours, thissolution was poured into 2 liters of water, and extraction was then madewith 1.5 liters of ethyl acetate. In succession, the resultant organiclayer was washed with 3N hydrochloric acid three times, a 2N aqueoussodium hydroxide solution three times and water. The resultant ethylacetate layer was dried over anhydrous sodium sulfate, and the solventwas then distilled off under reduced pressure. Next, the resultantcrystal was recrystallized from n-heptane twice to obtain 17.6 g (0.067mol) of 4-(trans-4-ethylcyclohexyl)phenoxypropanol. Its melting pointwas in the range of 68.5° to 69.9° C.

(b) Preparation of4-(trans-4-ethylcyclohexyl)phenoxypropyl!3,5-dinitrobenzoate

In a 500-ml three-necked flask equipped with a stirrer was placed 300 mlof THF, and 18.0 g (0.068 mol) of4-(trans-4-ethyl-cyclohexyl)phenoxypropanol obtained in theabove-mentioned manner and 11.4 ml of triethylamine were added, followedby stirring at 0° C. Afterward, a solution obtained by dissolving 15.7 g(0.068 mol) of 3,5-dinitrobenzoyl chloride in 50 ml of THF was addeddropwise to the flask over 30 minutes, and under this state, reactionwas carried out for 6 hours. After the completion of the reaction, thissolution was poured into 1 liter of water, and extraction was then madewith 1.5 liters of ethyl acetate. In succession, the resultant organiclayer was washed with 3N hydrochloric acid three times, a saturatedsodium bicarbonate solution three times and water. The resultant ethylacetate layer was dried over anhydrous sodium sulfate, and the solventwas then distilled off under reduced pressure. Next, the resultantcrystal was recrystallized from ethyl acetate twice to obtain 23.3(0.051 mol) of4-(trans-4-ethylcyclohexyl)phenoxypropyl!3,5-dinitrobenzoate. Itsmelting point was in the range of 108.0° to 109.4° C. The structure ofthis compound was confirmed by IR and NMR.

(c) Preparation of4-(trans-4-ethylcyclohexyl)phenoxypropyl!3,5-diaminobenzoate

In 300 ml of ethyl acetate was dissolved 15.0 g (0.033 mol) of4-(trans-4-ethyl-cyclohexyl)phenoxypropyl!3,5-dinitrobenzoate, and 1.50g of 5% palladium-carbon was then added thereto and catalytic reductionis carried out at ordinary temperature under atmospheric pressure. Afterthe completion of the reaction, the catalyst was removed by filtrationand the solvent was distilled off under reduced pressure to obtain acrystal. The thus obtained crystal was recrystallized fromn-heptane/ethyl acetate twice to obtain 11.1 g (0.028 mol) of4-(trans-4-ethyl-cyclohexyl)phenoxypropyl!3,5-diaminobenzoate. Themelting point of this compound was in the range of 91.5° to 92.4° C.

(3) Preparation of polyamic acid

In a 50-ml three-necked flask were placed 0.0606 g (0.15 mmol) ofstearyl 3,5-diaminobenzoate, 0.5348 g (1.35 mmol) of4-(trans-4-ethylcyclohexyl)phenoxypropyl!3,5-diaminobenzoate and 3.0 mlof N-methyl-2-pyrrolidone (NMP), and the mixture was stirred at roomtemperature under a nitrogen stream to dissolve them. Next, thissolution was maintained at 10° C., and 0.4413 g (1.50 mmol) of3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) was then addedthereto. Afterward, BPDA which adhered on the wall of the flask waswashed off with 3 ml of NMP, and under this state, reaction was carriedout for 3 hours to obtain a polyamic acid solution.

A part of this solution was poured into methanol, and the depositedprecipitate was then collected by filtration. Next, the collectedprecipitate was dried at ordinary temperature under reduced pressure toobtain a polyamic acid. The intrinsic viscosity of this polyamic acidwas 0.51 (30° C., 0.5 g/dl, an NMP solution).

(4) Polyimide and alignment film

The polyamic acid solution obtained in the paragraph (3) was diluted to4.0 wt % with an NMP/butyl cellosolve (1/1) solvent, and the dilutesolution was then applied onto a glass substrate equipped withtransparent electrodes of ITO by a spin coating method (a spinnermethod). After the application, the applied substrate was baked at 200°C. for 30 minutes to obtain the substrate provided with a polyimide filmhaving a thickness of 600 Å. This polyimide had a decompositiontemperature of 360.3° C. and good adhesive properties to the substrate.

Moreover, a part of the polyamic acid solution obtained in the paragraph(3) was diluted with NMP, applied onto a glass substrate, and thenheated at 200° C. for 30 minutes to obtain a film. The infraredabsorption spectrum (IR) of the obtained film is shown in FIG. 1.

(5) Preparation of cell and measurement of pretilt angle

The surfaces of two substrates on which polyimide films were formed weresubjected to a rubbing treatment, thereby forming liquid crystalalignment films. Next, a liquid crystal cell having a cell thickness of20 μm was constructed so that the rubbing directions of the liquidcrystal alignment films might be antiparallel to each other, and aliquid crystal ("FB-01", made by Chisso Corp.) was injected into thecell and it was then sealed. Afterward, the cell containing the liquidcrystal was subjected to a heat treatment at 120° C. for 30 minutes.After the heat treatment, the cell was allowed to stand, and at thistime, a measured pretilt angle was 29.2°. According to the observationof the cell by a microscope, an alignment state was not disordered, andthe cell exhibited an excellent alignment performance.

Examples 2 to 5

The same procedures as in paragraphs (3), (4) and (5) of Example 1 wererepeated except that each component ratio of stearyl 3,5-diaminobenzoateand 4-(trans-4-ethyl-cyclohexyl)phenoxypropyl!3,5-diaminobenzoate waschanged. The component ratios and the values of measured physicalproperties are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                                Copolymerization Ratio                                Formula  3!  Formula  4!                                                                              (molar ratio)                                         Example                                                                              R.sup.1   m     n   R.sup.2                                                                            Formula  1a!                                                                           Formula  2a!                         ______________________________________                                        1      C.sub.18 H.sub.37                                                                       3     1   C.sub.2 H.sub.5                                                                    0.1      0.9                                  2      C.sub.18 H.sub.37                                                                       3     1   C.sub.2 H.sub.5                                                                    0.3      0.7                                  3      C.sub.18 H.sub.37                                                                       3     1   C.sub.2 H.sub.5                                                                    0.5      0.5                                  4      C.sub.18 H.sub.37                                                                       3     1   C.sub.2 H.sub.5                                                                    0.7      0.3                                  5      C.sub.18 H.sub.37                                                                       3     1   C.sub.2 H.sub.5                                                                    0.9      0.1                                  ______________________________________                                                Polyamic Acid                                                                              Polyimide                                                        Logarithmic  Decomposition                                                                             Alignment Film                               Example Viscosity Number.sup.1)                                                                    Temp..sup.2)                                                                              Pretilt Angle                                ______________________________________                                        1       0.51         360.3       29.2°                                 2       0.39         358.2       38.1°                                 3       0.73         372.9       43.0°                                 4       0.55         349.9       22.6°                                 5       0.49         370.8       8.10°                                 ______________________________________                                         .sup.1) The logarithmic viscosity number was measured at 30° C. an     0.5 g/dl in NMP.                                                              .sup.2) Measurement was made at a temperature rise velocity of 15°     C./min, and a temperture at which weight reduction was 5% was regarded as     the decomposition temperature.                                           

Example 6

(1) Preparation of polyamic acid (A) (in the formula 2'!, m=3, n=1 andR² ═C₂ H₅)

In a 50-ml three-necked flask were placed 0.5942 g (1.50 mmol) of4-(trans-4-ethylcyclohexyl)phenoxypropyl!3,5-diaminobenzoate and 3.0 mlof an NMP solvent, and the mixture was stirred at room temperature undera nitrogen stream to dissolve it. Next, this solution was maintained at10° C., and 0.4413 g (1.50 mmol) of BPDA was added thereto. Afterward,BPDA which adhered on the wall of the flask was washed off with 3 ml ofNMP, and reaction was then carried out for 3 hours.

(2) Preparation of polyamic acid (B) (in the formula 1'!, R¹ ═H)

In a 50-ml three-necked flask were placed 0.6061 g (1.50 mmol) of1,3-phenylenediamine and 3.0 ml of N-methyl-2-pyrrolidone (NMP), and themixture was stirred at room temperature under a nitrogen stream todissolve it. Next, this solution was maintained at 10° C., and 0.4413 g(1.50 mmol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) wasadded thereto. Afterward, BPDA which adhered on the wall of the flaskwas washed off with 3 ml of NMP, and reaction was then carried out for 3hours to obtain a polyamic acid solution.

(3) Preparation of polyamic acid composition

With 1.00 g of the polyamic acid solution prepared in theabove-mentioned (1) were mixed 0.50 g of the polyamic acid solutionprepared in the above-mentioned (2), and the mixture was stirred, untila uniform state was attained, to obtain a polyamic acid compositionsolution.

(4) Liquid crystal alignment film comprising polyimide composition

The polyamic acid composition solution obtained in the above-mentionedparagraph (3) was diluted to 4.0 wt % with an NMP/butyl cellosolve (1/1)solvent, and the dilute solution was then applied onto a glass substrateequipped with transparent electrodes of ITO by a spin coating method (aspinner method). The applied substrate was baked at 200° C. for 30minutes to obtain the substrate provided with a polyimide compositionfilm having a thickness of 600 Å. This polyimide composition had adecomposition temperature of 391.4° C. and good adhesive properties tothe substrate.

The surfaces of the two substrates on which the polyimide compositionswere formed were subjected to a rubbing treatment, thereby formingliquid crystal alignment films.

Next, a liquid crystal cell having a cell thickness of 20 μm wasconstructed so that the rubbing directions of the liquid crystalalignment films might be antiparallel to each other, and a liquidcrystal (FB-01, made by Chisso Corp.) was injected into the cell and itwas then sealed. Afterward, the cell containing the liquid crystal wassubjected to a heat treatment at 120° C. for 30 minutes. After the heattreatment, the cell was allowed to stand, and at this time, a measuredpretilt angle was 16.7° . According to the observation of the cell by amicroscope, an alignment state was not disordered, and the cellexhibited an excellent alignment performance.

Example 7

(1) Preparation of polyamic acid (A)

A polyamic acid was prepared in accordance with the procedure of theparagraph (1) of Example 6.

(2) Preparation of polyamic acid (B) (in the formula 1'!, R¹ ═C₁₈ H₃₇)

In a 50-ml three-necked flask were placed 0.6061 g (1.50 mmol) ofstearyl 3,5-diaminobenzoate and 3.0 ml of an NMP solvent, and themixture was stirred at room temperature under a nitrogen stream todissolve it. Next, this solution was maintained at 10° C., and 0.4413 g(1.50 mmol) of BPDA was added thereto. Afterward, BPDA which adhered onthe wall of the flask was washed off with 3 ml of NMP, and reaction wasthen carried out for 3 hours.

(3) Preparation of polyamic acid composition

With 1.00 g of the polyamic acid solution prepared in theabove-mentioned (1) were mixed 0.11 g of the polyamic acid solutionprepared in the above-mentioned (2) to obtain a polyamic acidcomposition solution.

(4) Liquid crystal alignment film comprising polyimide composition

A liquid crystal alignment film was made in accordance with Example 6.The results are shown in Table 2.

Examples 8 to 11

The same procedure as in Example 6 was repeated except that a ratio of apolyamic acid solution to another polyamic acid solution was changed.The results are shown in Table 2.

A part of the polyamic acid solution obtained in Example 8 was dilutedwith NMP, applied onto a glass substrate, and then heated at 200° C. for30 minutes to obtain a film. The IR of this film is shown in FIG. 2.

                  TABLE 2                                                         ______________________________________                                                 Polyamic Acid B                                                                           Polyamic Acid A                                                   Formula  1'!                                                                              Formula  2'!                                             Example    R.sup.1       m     n       R.sup.2                                ______________________________________                                        6          H             3     1       C.sub.2 H.sub.5                        7          C.sub.18 H.sub.37                                                                           3     1       C.sub.2 H.sub.5                        8          C.sub.18 H.sub.37                                                                           3     1       C.sub.2 H.sub.5                        9          C.sub.18 H.sub.37                                                                           3     1       C.sub.2 H.sub.5                        10         C.sub.18 H.sub.37                                                                           3     1       C.sub.2 H.sub.5                        11         C.sub.18 H.sub.37                                                                           3     1       C.sub.2 H.sub.5                        ______________________________________                                              Composition     Polyimide  Alignment Film                               Ex-   Composition Ratio (molar ratio)                                                               Decomposition                                                                            Pretilt                                      ample Formula  1'!                                                                            Formula  2'!                                                                            Temperature.sup.1)                                                                     Angle                                      ______________________________________                                        6     0.5       0.5       391.4    16.7                                       7     0.1       0.9       319.4    5.21                                       8     0.3       0.7       339.3    7.25                                       9     0.5       0.5       335.1    13.3                                       10    0.7       0.3       363.4    18.2                                       11    0.9       0.1       373.5    20.4                                       ______________________________________                                         .sup.1) Measurement was made at a temperature rise velocity of 10°     C./min, and a temperture at which weight reduction was 5% was regarded as     the decomposition temperature.                                           

Example 12

(1) Preparation of polyamic acid (A) (in the formula 2'!, m=6, n=1 andR² ═C₃ H₇)

In a 100-ml three-necked flask were placed 0.6723 g (1.50 mmol) of4-(trans-4-propyl-cyclohexyl)phenoxyhexyl!3,5-diaminobenzoate obtainedin the same manner as in the paragraph (2) of Example 1 and 3.0 ml of anNMP solvent, and the mixture was stirred at room temperature under anitrogen stream to dissolve it. Next, this solution was maintained at10° C., and 0.4413 g (1.50 mmol) of BPDA was added thereto. Afterward,BPDA which adhered on the wall of the flask was washed off with 3 ml ofNMP, and reaction was then carried out for 3 hours to obtain a polyamicacid solution.

A part of this solution was poured into methanol, and the depositedprecipitate was then collected by filtration. Next, the collectedprecipitate was dried at ordinary temperature under reduced pressure toobtain a polyamic acid. The intrinsic viscosity of this polyamic acidwas 0.44 (30° C., 0.5 g/dl, an NMP solution).

(2) Polyimide

The polyamic acid solution obtained in the above-mentioned paragraph (1)was diluted to 4.0 wt % with an NMP/butyl cellosolve (1/1) solvent, andthe dilute solution was then applied onto a glass substrate equippedwith ITO by a spin coating method (a spinner method). The appliedsubstrate was baked at 200° C. for 30 minutes to obtain the a polyimidefilm. This polyimide film had a decomposition temperature of 377.1° C.

The IR of the obtained polyimide film is shown in FIG. 3.

We claim:
 1. A liquid crystal alignment film comprising a polyimide inwhich a logarithmic viscosity number of a polyamic acid as a precursoris in the range of 0.2 to 1.0 dl/g (in N-methyl-2-pyrrolidone solvent,concentration=0.5g/dl, 30° C.) and which contains a structural unitrepresented by the formula (2) ##STR15## wherein m is an integer of 1 to20; n is an integer of 1 or 2; and R² is an alkyl group or an alkoxygroup having 1 to 20 carbon atoms.
 2. A liquid crystal alignment filmcomprising a polyimide composition comprising a structural unitrepresented by the formula (S1) and a structural unit represented by theformula (S2) ##STR16## wherein m is an integer of 1 to 20; n is aninteger of 1 or 2; and R² is an alkyl group or an alkoxy group having 1to 20 carbon atoms; A is a divalent organic group; B is a tetravalentorganic group; and each α and β is a value which meets a logarithmicviscosity number of a polyamic acid as a precursor which is in the rangeof 0.2 to 1.0 dl/g (in N-methyl-2-pyrrolidone solvent, concentration=0.5g/dl, 30° C.).
 3. A liquid crystal display comprising the liquid crystalalignment film described in claim
 1. 4. A liquid crystal displaycomprising the liquid crystal alignment film described in claim
 2. 5. Aliquid crystal alignment film comprising a polyimide in which alogarithmic viscosity number of a polyamic acid as a precursor in therange of 0.2 to 1.0 dl/g (in N-methyl-2-pyrrolidone solvent,concentration=0.5 g/dl, 30° C.) and which comprises 5 mol % or more of astructural unit represented by the formula (2) and less than 95 mol % ofa structural unit represented by the formula (1) ##STR17## wherein m isan integer of 1 to 20; n is an integer of 1 or 2; R¹ is an alkyl grouphaving 1 to 20 carbon atoms; and R² is an alkyl group or an alkoxy grouphaving 1 to 20 carbon atoms.
 6. A liquid crystal alignment filmaccording to claim 2 wherein the formula (S1) is a structural unitrepresented by the formula (1') and the formula (S2) is a structuralunit represented by the formula (2') ##STR18## wherein m is an integerof 1 to 20; n is an integer of 1 or 2; R¹ is an alkyl group having 1 to20 carbon atoms; R² is an alkyl group or an alkoxy group having 1 to 20carbon atoms; and each of x and y is a value which meets a logarithmicviscosity number of a polyamic acid as a precursor which is in the rangeof 0.2 to 1.0 dl/g (in N-methyl-2-pyrrolidone solvent, concentration=0.5g/dl, 30° C.).
 7. A liquid crystal display comprising the liquid crystalalignment film of claim
 5. 8. A liquid crystal display comprising theliquid crystal alignment film of claim 6.